System for measuring loadings in a structure, measuring unit and measuring sensor

ABSTRACT

The invention relates to a system for measuring loading, stresses and/or material fatigue in a structure. The invention also relates to a measuring sensor unit ( 50 ) and a measuring sensor ( 302 ) suitable for use in connection with said method. The invention is applicable especially to the measurement of stresses and loading on a ship hull. One inventive idea is that the measuring sensor unit ( 50 ) of the measuring system comprises means ( 310 ) for processing a signal from the sensor so as to allow complete mathematical measuring results to be transmitted ( 316, 21 ) from the measuring sensor unit to the central processing unit. Another inventive idea is to form a measuring sensor from the sensor assembly and the strain gauge so that deformations are transmitted to the strain gauge attached to an elastic area of the sensor assembly. By means of the invention, the measuring units can be calibrated apart from the structure material and additional measuring units can be provided in the system whenever necessary.

The invention relates to a system for measuring loading, stresses and/ormaterial fatigues occurring in a structure. The invention also relatesto a measuring unit and a measuring sensor suitable for use inconnection with said system. The invention is applicable especially tothe measurement of stresses and loading in a ship hull.

Load exerted on a ship hull must be continuously measured for anychanges taking place in the structure, such as material fatigue, to bedetected before the structures break. Similar measurements are performedregarding other structures, such as other vehicles, bridges andbuildings.

There are conventional methods comprising the fastening of measuringsensors to a ship hull to measure local deformations of the structure.FIG. 1 shows a top view of the main deck 10 of a ship hull and thetypical positioning of measuring sensors 11, 12, 13 and 14 on the maindeck of the ship hull. Typically used measuring sensors are steel barswith an approximate length of 1-2 meters, which are solidly attachede.g. by welding to the main deck of the ship hull, with one end of thebar stationary and the other end axially movable. At the junction, e.g,.a movement sensor is disposed to measure the mutual movement. The bar ofthe measuring sensors shall have a length at least equal to the lengthmentioned above for the relative movements of the bars to besufficiently large (typically a maximum of +/−5 mm) and the deformationmeasurement to be sufficiently accurate.

In conventional methods, the weak signal obtained from the measuringsensors is conducted to the central unit of the measuring system, wherethe signals received from the sensors are amplified and converted intodigital form, and mathematical signal processing is performed. There aredifferent standard signal processing models to allow optimally reliabledetection of critical deformations by means of measurement results. Suchmodels are i.a. the frequency range analysis and the so-called Rainflowclassification. The Rainflow classification is defined i.a. in ASTMStandard E1049-85 (Reapproved 1990).

The prior art systems mentioned above involve some drawbacks. Firstly,the systems have been devised for a given number of standard measuringsensors, so that additional measuring sensors cannot be provided ifnecessary. This is due to the fact that the computing capacity of thecentral processing unit of the system and the interfaces have beendimensioned for a predetermined number of measuring sensors. There aresituations where the usual number of measuring sensors will not besufficient, such as for instance in a catamaran ship, where aconsiderable greater number of measuring sensors, e.g. 60 units, mightbe necessary, compared with a “conventional” ship hull, where there areusually e.g. 4 measuring sensors.

Secondly, prior art systems involve the problem of interferenceconnected to the long measuring lines between the measuring sensors andthe central processing unit. This is difficult to avoid by protectingthe measuring cables, because the signals to be transmitted from themeasuring sensors are low-frequency analogue signals.

In addition, prior art measuring sensors also involve the problem ofonerous calibration. On reason for this is the cumbersome detachment oflarge measuring sensors from the ship hull, and another reason is that alarge-sized measuring sensor may behave differently when attached to aship hull than under calibration conditions. Consequently, accurate andreliable calibration will be difficult to achieve. The replacement of adefective measuring sensor is also awkward.

The object of the present invention is to avoid the prior art problemsmentioned above with the aid of a new measuring solution.

One inventive idea is that the measuring sensor unit, i.e. measuringunit of a measuring system, comprises means for processing a signalobtained from the sensor so that complete mathematical measurementresults can be transmitted from the measuring unit to the centralprocessing unit. In this manner, no large computing power will berequired in the central processing unit, nor does this capacityrequirement depend substantially on the number of measuring sensorsused. Thus the number of measuring sensors can be readily increased. Inaddition, the negative impact of interference connected to the cablebetween the measuring unit and the central processing unit will besmall, because the signals to be transmitted are digital and thetransmission can be repeated if necessary.

A second inventive idea is that the measuring sensor has mechanicalfeatures allowing it to be readily attached to and detached from thestructure to be measured. In this case, the structure to be measuredpreferably includes a base for attachment of the measuring sensor. Themeasuring sensor can then be calibrated apart from the structure to bemeasured and the behaviour of the measuring sensor will be exactly thesame in the structure to be measured as under calibrating conditions.

A further inventive idea is to form a measuring sensor from a sensorassembly and a strain gauge so that deformations are transmitted to thestrain gauge, which is attached to an elastic area in the sensorassembly. The elastic area is preferably formed with a double H -openingin the sensor assembly. In addition, the sensor preferably comprises asecond strain gauge for temperature calibration, which is attached to apart of the sensor assembly where no deformation occurs. In this manner,a small-sized but high-precision measuring sensor will be provided,which is easy to handle e.g. during calibration.

The system of the invention for measuring loading on a structure, whichcomprises a central processing unit and at least one measuring unit, ischaracterised by the measuring unit comprising

a measuring sensor for converting structure deformations into anelectric signal,

means for converting said signal into a digital signal,

means for mathematical processing of the digital signal, and

means for transmitting the processing results to the central processingunit, the central processing unit comprising means for receiving andcollecting processing results transmitted from at least one measuringunit.

The measuring sensor of the invention for measuring loading on astructure is characterised by the fact that the measuring sensorcomprises a sensor assembly, which is attached to the structure to bemeasured and includes two rigid members for attachment to the structureto be measured and an elastic member between these, a first strain gaugehaving been fastened to said elastic member for transmittingdeformations of the structure and the sensor assembly to the straingauge with a view to generate a signal proportional to the deformationsof the strain gauge.

The measuring unit of the invention for measuring loading on a structureis characterised by the fact that the measuring unit comprises

a measuring sensor for converting deformations of a structure into anelectric signal,

means for converting said signal into a digital signal,

means for mathematical processing of the digital signal, and

means for transmitting the processing results to the central processingunit. Preferred embodiments of the invention are described in thedependent claims.

The invention is explained in greater detail below with reference to theaccompanying drawings, in which

FIG. 1 shows conventional positioning of measuring sensors on the deckof a ship hull,

FIG. 2 is a block diagram of a measuring system of the invention,

FIG. 3 is a block diagram of a measuring unit of the invention,

FIG. 4 is a flow diagram of a procedure of the invention fortransmitting measuring data,

FIG. 5a is a top view of a measuring sensor of the invention and

FIG. 5b is a lateral view of a measuring sensor of the invention.

FIG. 1 has been explained above in connection with the description ofprior art.

FIG. 2 is a block diagram of a measuring system of the invention. Itcomprises a central processing unit 20, which is connected to a digitaldata communication bus 21. Connected to the data communication bus 21,there is a plurality of measuring units 30, 32 and 34, which are placedat suitable measuring locations in the structure to be measured.

The central processing unit may comprise e.g. a PC-based computer withe.g. an NT operating system. In that case, in a windows NT operatingsystem, a divided memory may act as an open user interface, over whichthe user has access to the measuring results in the system and cancontrol the system. The data communication bus may preferably be anARCNET-data network (Attached Resource Computer Network). An ARCNETcomputer network has the feature of automatic reconfiguration. Thisfeature is preferably utilised by performing an addition and a removalof a measuring sensor/unit without any program modifications or othermodifications of the system. Each measuring sensor/unit is identified inthe system on the basis of its own individual identifier (e.g. serialnumber), so that the data are organised to the user interface (datastructure) in the divided memory of the Windows NT by the individualidentifier. The ARCNET computer network automatically handles the datacommunication node identifiers in connection with each configurationactivation. Hence cross-connection incorporated in the messageinterpretation of the central processing unit can be automaticallyorganised and connect the data communication node identifier to theindividual identifier of the measuring unit. Configuration isconsequently not needed as a user operation.

FIG. 3 shows a measuring unit 30 of the invention. It comprises aWheatstone bridge 302, in which a first strain gauge for measuringdeformation and a second strain gauge for temperature compensation areserially connected. Stationary resistors form the second part of thebridge.

A measuring signal obtained from the Wheatstone bridge is amplified withthe instrumentation amplifier 304, whose output signal is furtherfiltered with a low pass filter 306. The measuring frequency band ise.g. 0-150 Hz, and the frequency band can preferably be selected byprogramming. The amplified and filtered analogue signal is convertedinto a digital signal with an analogue-digital (A/D) converter 308, fromwhere the digital samples obtained are directed to the microprocessor310. The A/D converter 308 may also be included in the processor 310.The processor 3 10 stores the measuring results in the storage 314 overthe internal bus 312 in the measuring unit. This repeatedly programmablestorage 314 has a capacity of e.g. 4 kB+32 kB. For instance, a real-timemeasuring signal covering the last 10 seconds can be stored in thememory. Computing parameters can also be stored in the memory. The datacommunication interface 316 connected to the bus 312 furthercommunicates with the system bus 21, which transmits data between thecentral processing unit and the measuring units. The transmission ofmeasuring data from the measuring unit memory 314 to the centralprocessing unit takes place on the basis of commands from the centralprocessing unit and a program stored in the measuring unit. At therequest of the central processing unit, the measuring unit can e.g.transmit a 10 second sample history to the central processing unit.

The measuring unit transmits a measurement sample signal to the centralprocessing unit at a sample frequency, which is preferably lower thanthe sampling frequency of the D/A-converter. The measuring unit can alsobe configured over the data communication bus. Thus, for instance, it ispossible to choose the analysing methods programmed in the measuringunit to be implemented. The measuring unit may also transmit itsconfiguration setting at determined intervals to be checked by thecentral processing unit. Also, the measurement offset can be calibratedand the data sampling frequency can be set under the control of thecentral processing unit. The measuring unit also preferably supervisesits own operation. Thus, for instance the cycle of signal analysingprograms can be supervised in order to prevent overload on the computingcapacity of the processor.

The processor 310 comprises preferably an internal permanent ROM memory,which may have a capacity of e.g. 128 kB. The programs relating both tothe measuring unit functions and those relating to data communicationbetween the central processing unit and the measuring unit are stored inthis ROM. The required signal analysing algorithms are also stored inthe ROM, the algorithms being e.g.:

calculation of the signal average,

calculation of the effective signal value,

expressions of peak values: a positive peak value, a negative peak valueand the maximum peak-to-peak value,

calculation of zero overflows and average zero overflow frequency,

Rainflow classification and

calculation of the frequency range spectrum of the signal by the FastFournier (FFT) process.

These analysing methods programmed in the processor of the measuringunit are preferably implemented by the m measuring unit in real time.

In addition to the blocks mentioned above, the measuring unit maycomprise circuits for generating operating voltages from the supplyvoltage obtained from the bus (not represented in FIG. 3).

FIG. 4 shows a flow diagram of a procedure 400 in accordance with theinvention for transmitting data between a measuring unit and the centralprocessing unit. First, the measuring unit checks whether a message hasbeen received from the central processing unit, step 402. If a messagehas been received, it is interpreted, 404. Then follows a check whetherthe measuring unit has received new configuration data, step 406. If newconfiguration data have been received, the configuration of themeasuring unit is reset according to these new data, 408. Next follows acheck whether the measuring unit has received a command to transmit thevalid configuration data to the central processing unit 410. The commandmay be e.g. a bit/bit string to this effect in a message addressed bythe central processing unit to the measuring unit. The command may alsoderive from a sensor timer. If such a command has been received, theconfiguration data are sent to the central processing unit, 412.

Then it is checked whether the measuring unit has received a command tosend mathematically calculated measuring data to the central processingunit, step 414. If such a command has been received, the measuring dataare transmitted to the central processing unit, 416. After this followsa,check whether the measuring unit has received a command to send signalhistory to the central processing unit, 418. If such a command has beenreceived, the signal history is sent to the central processing unit,420. Then it is checked whether the measuring unit has received acommand to send a Rainflow classification to the central processingunit, 422. If such a command has been received, the Rainflowclassification is sent to the central processing unit, 424. Then it ischecked whether the measuring unit has received a command to send FFTresults to the central processing unit, 426. If such a command has beenreceived, the FFT results are sent to the central processing unit, 428.Finally follows a check whether the signal sample buffer of themeasuring unit contains data, step 430. If the buffer contains data, thedata are sent to the central processing unit, 432. In that case, thetransmission preferably takes place at a sample frequency lower than thesampling frequency. At the end of these steps, step 402 is resumed, andthe steps mentioned above are repeated, so that the measuring dataneeded for the central processing unit in each case are actually sent tothe central processing unit in conformity with the commands of thecentral processing unit. It should be noted that apart from the centralprocessing unit, the commands mentioned above may consequently derivefrom the measuring unit timer.

FIGS. 5a and 5 b show a measuring sensor 50 of the invention, which issuitable for use i.a. in the measuring unit of the invention. FIG. 5ashows a top view of the measuring sensor and FIG. 5b a side view of themeasuring sensor. The measuring sensor comprises a sensor assembly 502having rigid members 504 and 508 at its ends and an elastic member 506at its centre. The rigid and elastic members are separated with a brokenline in the figure. The rigid member of the sensor assembly has holes520 and 522 for attachment of the sensor to the structure material to bemeasured or to mounting members provided in the structure material.

The elastic member 506 of the sensor assembly has two openings 510 and512 in the shape of an H, i.e. forming a so-called double H opening.With the aid of this opening, the elastic member has been impartedhigher elasticity than the rigid member. Between the two H-shapedopenings, a first strain gauge sensor 530 has been attached, which,connected e.g. to a Wheatstone bridge, provides a signal proportional tothe deformation of the elastic member. In addition, within the second Hopening, a second strain gauge sensor 532 has been attached, in whosemounting base no deformation occurs. The purpose of this second straingauge sensor is to act as a reference resistor of the Wheatstone bridgeand to compensate for the resistance changes caused by temperaturevariations of the sensor in the first strain gauge sensor.

The elastic area of the sensor assembly material (i.e. a limit ofelasticity of σ_(E)) is preferably selected so as to be larger than thatof the structure material, for the structure material deformations to bemeasured not to cause permanent changes in the elastic area of thesensor assembly. Moreover, the sensor assembly material has beenselected so as to resist varying loads without fatigue, in the way thestructure material does. The legs in the double H-shape of the sensorassembly are rectangular cross-sectional surfaces, whose length does notexceed the buckling length under compression load when the compressionload corresponds to the deformation of the structure material over itsentire elastic area (σ_(E)).

The sensor is preferably fastened by bolts to mounting members welded tothe structure material, so that the measuring sensor can be replaced andcalibrated. In fact, the measuring sensor can be mounted as normalprecision work. The power transmission from the structure material takesplace by means of a frictional joint, the adhesive surfaces of thesensor assembly having been roughened with cobalt carbide coatings 544and 548. The carbide crystals of the coating penetrate into matingsurfaces of the fastening elements welded to the structure material,thus preventing sliding.

When the measuring unit comprises the sensor of the invention and thesignal processing electronics of the invention, the unit can beencapsulated preferably with the casing bottom comprising the measuringsensor and the cover part of the casing comprising the signal processingelectronics. In addition, the bottom part and the cover part arepreferably joined in a way which ensures a water-proof unit. A fixedcable extendable in an appropriate junction box is preferably used inthe measuring unit.

A second option is encapsulating the measuring sensor and the signalprocessing electronics in discrete casings. Such a solution may beadvantageous when it is necessary to carry out mounting of the sensorpart under explosive circumstances. Such locations are e.g. the innersurfaces of various containers. The measuring electronics is thenpreferably placed outside the container, for instance.

A number of preferred embodiments of the invention has been describedabove. The principle of the invention can naturally be varied within thescope of protection defined by the claims with regard to details andfields of application.

It should be especially noted that, besides in the measurement of theloading on a ship hull, the invention can be applied to most variedstructures. It should also be noted that the measuring sensor, measuringunit and measuring method and/or system of the invention can also beimplemented independently of each other.

What is claimed is:
 1. A measuring sensor for measuring loading on astructure, comprising: a sensor assembly which is configured to beattached to the structure to be measured and comprises two rigid partsconfigured for attachment to the structure to be measured and an elasticpart between the two rigid parts, a first strain gauge attached to theelastic part for transmitting deformations of the structure and thesensor assembly to the strain gauge to generate a signal proportional tothe deformations, a second strain gauge for temperature compensationwhich is attached to a portion of the sensor assembly where nosubstantial deformations occur.
 2. A measuring sensor as defined inclaim 1, wherein said first strain gauge and second strain gauge areintegrated in a Wheatstone bridge.
 3. A measuring sensor as defined inclaim 1, wherein said sensor assembly comprises a double-H opening forforming an elastic area and providing a measuring point for said firststrain gauge.
 4. A measuring sensor as defined in claim 1, wherein anelastic area/limit of elasticity of material of the sensor assembly isselected so as to be higher than an elastic area/limit of elasticity ofmaterial of the structure.
 5. A measuring sensor as defined in claim 1,wherein surfaces on the rigid parts bearing against the structure oragainst fastening members provided in the structure have been roughened.6. A measuring sensor as defined in claim 3, wherein said first straingauge is attached between two H openings.
 7. A measuring sensor asdefined in claim 3, wherein the second strain gauge is attached to acentral member of the H opening.